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FOSSIL   JAWS                                        97

have resisted the decay which would more readily affect the
softer "bones. Where there are bones it is frequently the lower
jaw alone which has been preserved for usa bone which has also
been preserved in the case of some of the contemporary Marsupials.
It has been pointed out (from the observation of dead dogs
floating in canals) that the lower jaw is occasionally detached
from the carcase. It is the most readily separable part which
contains a skeleton. It may be, therefore, that the remains of
these early mammals, floating down some river to the sea, may
have lost their jaws while in the river, or at furthest in the
shallow waters of the sea, the rest of the carcase floating out to
a greater distance, and being finally entombed in the stomach
of some carnivorous fish, or in the mud at the bottom of a deep
ocean, which has never since seen the light.
The characters of this group are really more those of the
Monotremata than of the Marsupials. The undoubted likeness
which their molar teeth show to the temporary teeth of the
Platypus have already been commented upon. Like the Mono-
tremes the Allotheria appear to have possessed a large and
independent coraeoid; the evidence for this rests upon the
discovery of the lower end of a scapula of Camptomus, a Cretaceous
genus from North America upon which there is a distinct facet
for the articulation of what can have been nothing else than a
coraeoid. On the other hand they differ from the Monotremata
by the presence of incisor teeth which were E,odent-like in form,
and not very different from those of certain Marsupials. This
point of difference cannot be regarded as of very first-rate im-
portance ; no one would relegate the Sloth and the Armadillo to
different orders on account of their tooth differences, which are
about on a par with those to which we have just referred. It
seems indeed likely that it will be ultimately necessary to rub
out the boundary line which now divides the Allotheria and the
The Plagiaulacidae are unquestionably mammals, and they
are placed by most naturalists in this at present uncertain group
of Multituberculata, which will be retained here in deference to
the distinguished authorities who have instituted the group,
though there are but few characters by which it can be defined.
This family though appearing in the Trias, extends down in time
to the Eocene. The type-genus, that which lias given its name to
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